1. Field of the Invention:
The present invention relates to heat-developable photosensitive materials and particularly to heat-developable photosensitive materials having preferred film properties in any stage of storage, treatment and use.
2. Description of the Prior Art:
Photographic processes using silver halide have been widely utilized hitherto, because they are excellent in photographic properties such as sensitivity or gradation as compared with electrophotographic processes or diazophotographic processes. Recently, various arts capable of obtaining images simply and rapidly have been studied and developed, where the image forming treatment of silver halide photosensitive materials is converted from a wet treatment with a developing solution into a dry treatment by heating.
At present, the most successful art in the field of sensitive materials capable of forming photographic images by dry treatment includes heat-developable photosensitive materials utilizing a composition comprising an organic silver salt, a small amount of a photocatalyst and a reducing agent as essential components as described in U.S. Pat. Nos. 3,152,904 and 3,457,075. When these photosensitive materials which are stable at a normal temperature are heated to 80.degree. C. or more and preferably 100.degree. C. or more after image exposure, the organic silver salt oxidizing agent in the photosensitive materials enters an oxidation-reduction reaction as a catalytic function of an exposed photocatalyst existant in the vicinity thereof to form silver, by which exposed areas of the sensitive layer rapidly blacken to form contrast between non-exposed areas (background) and, consequently, images are formed.
In this photosensitive system, though a process which allows the residual photocatalyst in the photosensitive materials after development to discolor by light is adopted, the same effect as that in case of carrying out stabilization is obtained. Since the amount of photocatalyst used is small and the greater part thereof is a white or lightly colored organic silver salt, which is difficult to blacken, even though a small amount of the photocatalyst (for example, silver halide) discolors, such slight discoloration is hardly detectable by the eyes.
The above-described heat-developable photosensitive materials are generally produced by applying a sensitive layer containing the above-described organic silver salt, a photocatalyst, a reducing agent and a binder to a support.
In the known heat-developable photosensitive materials having such a structure, the sensitive layer often adheres to heating means or suffers damage when they are brought into contact with the heating means upon thermal development. Particularly, problems arise in the case of thermal development by a heating drum; namely, the binder softens to cause separation of part of the sensitive layer and the images shift in the revolving direction of the drum. This can be prevented to some degree by providing a heat resisting top coating polymer layer on the sensitive layer, but shearing of the images can not be prevented by providing only a top coating polymer layer. Further, experience has taught that a better result is obtained when the polymer in the top coating polymer layer is different than that in the sensitive layer. In this case, however, differences in polarity (the degree of hydrophilic or oleophilic property) between the sensitive layer and the top coating polymer layer weaken other adhesion between the layers, making them susceptible to scratching or separation of the layer by adhesion (for example, a pressure-sensitive adhesive tape) during processing, etc., frequently result.
On the other hand, an undercoating layer is sometimes provided between the sensitive layer and the support in order to improve storage stability of the heat-developable photosensitive materials under high humidity as described in Japanese patent application No. (OPI) 87721/78. However, due to the difference in polarity between the polymer in the undercoating layer and the polymer in the sensitive layer, separation of layers, etc. easily result. Further, when using the particularly useful vinyl chloride type copolymer or vinylidene chloride type copolymer as the undercoating layer, it is not possible to use a thermal development apparatus in which the heating member is substantially sealed from the outside (e.g., a combination of a heating roll and a heating shoe), because moisture contained in equilibrium in the base evaporates upon heating to deform the undercoating layer, and undesirable bubbles are formed on the images.
Simultaneous multilayer application processes such as described in U.S. Pat. No. 2,761,791 have the advantages of reducing poduction cost and increasing the production rate, but to use these processes, a similar solvent, composition should be used for each layer and the same polymer is preferably used as a binder in each layer in order to balance surface tension. However, when polyvinyl butyral, widely used as a binder for sensitive layers of heat-developable photosensitive materials, is used for the top coating layer, it easily adheres to the developing apparatus by heat. Accordingly, up to now, it has been difficult to carry out multilayer simultaneous application in the field of the heat-developable photosensitive materials.
The present invention is based on the discovery that inferior film properties observed in the prior heat-developable photosensitive materials can be improved using polyisocyanates.
U.S. Pat. No. 3,846,136 discloses that the photosensitive heat-sensitive layer or other hardenable layers in heat-developable photosensitive materials can be hardened with organic or inorganic hardening agents such as aldehydes and blocked aldehydes, ketones, carboxylic and carbonic acid derivatives, sulfonate esters, sulfonyl halides and vinyl sulfonyl ethers, active halogen compounds, epoxy compounds, aziridines, active olefins, isocyanates, carbodiimides, mixed-function hardeners and polymeric hardeners such as oxidized polysaccharides like dialdehyde starch and oxy-guargum and the like. However, this disclosure is merely an invitation to experiment, because almost all hardening agents known for gelatino silver halide emulsion photographic-sensitive materials processed by wet treatment are enumerated and there is no example using them. In fact, according to examination by the present inventors, many of the above-enumerated hardening agents are unavailable or can not be used for the heat-developable photosensitive materials because they have ill effects on the heat-developable photosensitive materials as described in the following table. Further, the above-described isocyanates have not been used in conventional systems because of their inferior stability. Accordingly, it is difficult to predict from the description of U.S. Pat. No. 3,846,136 which hardening agents are practically effective or what kind of effect will be shown in heat-developable photosensitive materials which are different from the conventional gelatino silver halide emulsion photographic sensitive materials in binder and treatment.
Further, epoxy compounds described in U.S. Pat. No. 3,846,136 harden heat-developable photosensitive materials, but they have a tendency to increase discoloration by room light (light discoloration) after development. Further, carbodiimides have high activity and hardening ability, but they have a high physiological function and they are not preferred from the viewpoint of the public nuisance. Therefore, both are problems in use.